The present invention relates to cellular communications networks and, in particular, to a novel downlink slot structure and method of power control for use in a TDMA packet data network.
In order to enable multiple users of a cellular network to share the same geographic region, or xe2x80x9ccellxe2x80x9d, a time-division multiple-access (TDMA) standard was developed by members of the Telecommunications Industries Association, which standard came to be known as IS-136 and which is incorporated by reference herein. According to IS-136, time is divided into frames of 40 milliseconds (ms) and each frame is further divided into slots of 6.67 ms each. In the version of IS-136 currently implemented by most wireless service providers, a circuit is established between a base station and a mobile unit by using two or more slots out of each frame.
This circuit-switched characteristic of most current IS-136-compliant cellular systems works well when the only type of information to be communicated is voice information and when bandwidth is abundant. However, a growing number of communications applications currently in use today involve the transfer of data packets, common examples being Web browsing, file transfers, electronic commerce and electronic mail. As opposed to voice exchanges, packet transfers are sporadic, asynchronous and have varying bandwidth demands. It is therefore highly inefficient to establish a dedicated circuit for channelling a packet transfer between a base station and a mobile unit when the circuit may service the occasional data burst but otherwise remains idle.
Thus, it would be desirable to apply the TDMA concept to a packet-based architecture, although without excessively deviating from the current TDMA standard. To this end, members of the TIA are in the process of developing a new TDMA standard, known as GPRS-136. GPRS-136 is a packet delivery service that is essentially based on the GPRS (general packet radio service) protocol as defined by the ETSI (European Telecommunnications Standards Institute) but specifically adapted to operate on an IS-136 air interface. Therefore, GPRS-136 continues to provide slots of 6.67 ms and frames of 6 slots each, as in conventional IS-136.
Nevertheless, while certain similarities between GPRS-136 and IS-136 are present for reasons of compatibility, GPRS-136 also allows the configuration of new slot structures which differ significantly from the traditional IS-136 model. This permits the design of slot structures that counter various disadvantages of current TDMA systems which are compounded by the packet-based nature of today""s traffic mix, while remaining within the broad scope of the accepted IS-136 air interface standard.
For instance, co-channel interference is a phenomenon known to affect a mobile unit when it enters a geographic region in which the signals from multiple base stations are strong enough to interfere with each other. In a packet-based system, out of the many mobile units communicating with base stations that contribute to the co-channel interference, it is likely that one or more will be idle and will not require the transmission of packets at any given time instant. Co-channel interference could therefore conceivably be mitigated by reducing the power emitted by a base station while in communication with idle mobile units.
However, if the current IS-136 slot structure and in particular the digital control channel (or DCCH) were adopted for packet transmission, it would be extremely impractical to achieve a reduction in co-channel interference. The reason for this is that the DCCH slot structure contains control fields that are distributed among the data fields of the slot, but these same control fields comprise information that must be properly received by the mobile unit, even if the data fields contain no valid data, e.g., when the mobile unit is idle.
Thus, a base station wishing to reduce co-channel interference should execute fast and frequent power ramps to keep the transmitted power at a high level for the control fields, while possibly lowering the power during the (fragmented) data fields of the slot. Unfortunately, an ordinary base station transmitter cannot provide such rapid variations in output power as are called for by the nature of the currently used IS-136 slot structure.
Clearly, there is a need for a new slot structure which is better suited to a packet-switched communications architecture and, specifically, which possesses advantageous properties to allow the transmitted power to be reduced during the transmission of null slots, thereby to reduce co-channel interference.
It is an object of the present invention to mitigate or obviate one or more disadvantages of the prior art.
Therefore, in accordance with a first broad aspect, the invention provides a novel slot structure for use in a TDMA communications system. The slot comprises a header, a data field adjacent the header and a ramp interval adjacent the data field.
The invention may be summarized according to another broad aspect as a method of transmitting data at a controllable power level. The method comprises arranging the data into slots having the just described structure and then choosing one of two power curves for transmitting a given slot. If the slot is a null slot, then the header is sent at a high power and the power is ramped down to a low power during the first few symbols of the data field. The remainder of the data field is transmitted at this low power until the power is ramped back up again during the ramp interval.
Because the header is separate from the data field, it becomes practical to implement the power control mechanism using currently available base station transmitters, with the result that power is conserved at the base station transmitter and co-channel interference is mitigated.